1. Field of the Invention
The present invention relates to a lithography process, particularly to a lithography process which is suitable for producing an high density integrated circuit.
2. Related Background Art
Semiconductor elements are becoming more and more highly integrated: LSI's of 0.2-0.3 .mu.m are now being investigated. Accordingly, it has been required in lithography to form fine resist patterns having a processing latitude. Hence, photoresists having a high resolution are used to form the patterns. Such resists are of a positive type or a negative type. In usual instances, the patterns are formed using an alkaline developer as exemplified by a 2.38% by weight aqueous solution of tetramethylammonium hydroxide (hereinafter "TMAH").
As resist materials have been made to have a higher resolving power, the ratio of dissolving rate at exposed areas of a resist layer to that at unexposed areas thereof, i.e., the selection ratio of a resist to a developer has been improved. The state thereof is shown in FIG. 26. In FIG. 26, the ratios of dissolving rate at exposed areas of a resist layer to that at unexposed areas thereof are shown as functions of TMAH concentration. In the drawing, a positive photoresist A has a resolution of 1.2 .mu.m, a positive photoresist B, 0.8 .mu.m, and a positive photoresist C, 0.6 .mu.m. As shown in the drawing, the selection ratio a resist has is seen to have exponential-functionally increases with an improvement in the resolution of the resist. In other words, an achievement of a resist having a high resolution is seen to be equal to an improvement in the selection ratio at exposed areas of a resist layer and unexposed areas thereof.
In order to improve developability, it is attempted to increase the concentration of a developer. Such an attempt, however, leads to a decrease in the selection ratio at exposed areas of a resist layer and unexposed areas thereof, resulting in a deterioration of the resolution performance of resists.
FIG. 27 shows a 0.55 .mu.m line-and-space resist pattern developed by a developer of a conventional type in which no surfactant is added. Exposure was carried out using a g-line reduction aligner having a lens with a numerical aperture of 0.43. Because of a poor wettability of the developer on the resist surface, trailing and residues (shown by the encircled portions, 2401, in FIG. 27) of the resist pattern are seen to have occurred after development. In order to prevent such a phenomenon, a developer containing a surfactant has been used in some instances. When, however, some types of substrates are used, none of conventional developers containing a surfactant can prevent the resist residues from occurring. Conversely, the addition of some types of surfactants has caused a decrease in the sensitivity of resists or a decrease in exposure latitude.
FIG. 28 shows the flatness of the surface of a silicon wafer having been immersed for 60 seconds in the developer of a conventional type in which no surfactant is added, which flatness is indicated as the central-line average roughness (JIS B0601). As is seen therefrom, the wafer having been immersed in the developer has a very rough surface only because of its contact with the developer for a time as short as 60 seconds, compared with a reference wafer having not been immersed in the developer. Fabrication of semiconductor devices in such a state of surface brings about an extreme deterioration of electrical characteristics. FIG. 29 shows data obtained by examining a channel mobility of MOS transistors fabricated by the use of wafers having a rough surface. The channel mobility is seen to be extremely poor with respect to the reference value when the surface has roughed upon its contact with the developer.
The silicon surface can be prevented from roughing when a certain surfactant is added to the developer, but the surfactant is adsorbed onto the silicon. FIG. 30 shows data obtained by observing the carbon 1s peak by X-ray photoelectron spectroscopy (XPS). A peak 2701 that shows the presence of the surfactant was seen in the data from the silicon surface exposed for 60 seconds to the developer containing a surfactant. It was found that, when the developer containing a surfactant was used, the surfactant was adsorbed to the underlying substrate and was not removable by usual rinsing. Adsorption of the surfactant to the substrate surface caused a carbon contamination to bring about a lowering of film quality in the subsequent formation of various kinds of thin films. On the other hand, when a surfactant not adsorptive to the silicon surface was selected, the surface of silicon roughed.
Under existing circumstances, no resist can show its ability when contact holes are formed using a resist having a resolution up to 0.6 .mu.m (capable of forming line-and-space patterns with a mask fidelity).
FIGS. 31A and 31B shows cross-sectional photographs of contact holes developed using a conventional developer. In the case of exposure conditions under which contact holes with a size of 0.8 .mu.m are formed (220 mJ/cm.sup.2), 0.7 .mu.m and 0.6 .mu.m holes are in a condition of under-exposure.
In the case of exposure conditions under which contact holes with a size of 0.7 .mu.m are formed (280 mJ/cm.sup.2), 0.8 .mu.m holes are in a condition of over-exposure and 0.6 .mu.m holes are in a condition of under-exposure. Namely, in regions having a smaller exposure area in themselves (e.g., hole regions), the developer had so poor a wettability that it did not penetrate into the pattern and hence no satisfactory resist performance was exhibited.
In order to improve the wettability of developers, developers containing a surfactant are used. FIG. 32 shows the relationship between the concentration of a surfactant added to a developer and the exposure threshold energy for the formation of contact holes. The contact hole exposure threshold energy is meant to be an exposure energy necessary for the bottom of a contact hole to reach the underlying substrate. The exposure threshold energy decreases with an increase in the concentration of the surfactant added to the developer, but the difference in exposure threshold energy between different hole sizes does not change from the state before the addition of the surfactant. Namely, when conventional surfactants are used, it has been impossible to form fine contact holes with different size, in a high precision under the same exposure conditions.
FIG. 33 shows cross-sectional photographs of contact holes formed using developers containing the surfactant as shown in FIG. 32. FIG. 33 shows that when the conventional surfactants are added the side walls of contact holes have a curved shape. When the amount of a conventional surfactant added was increased to improve the wettability of the developer, a deterioration of hole configuration occurred.
In addition to the above discussed high-precision development of resists, a method of exposure in high resolution is indispensable for forming an extremely fine pattern precisely as designed, and is now being investigated actively.
Methods conventionally employed for improving the resolution of the apparatus is to increase the NA of an optical system (demagnification projection lens system). However, the increase of NA gives rise to a problem of decrease of the focal depth because the focal depth is inversely proportional to the square of NA. Accordingly, in recent years, change of the exposure light length is attempted from the g-line to the i-line or excimer laser of 300 nm or shorter wavelength, which utilizes the effect that the focal depth and the resolution of an optical system are improved in inverse proportion to the wavelength.
A further method which has recently been developed for improving the resolution of the exposure is use of a phase shift mask. In this method, a thin film which causes a phase shift of 180.degree. relative to other part is formed in a partial area of the light-transmissive portion of the mask. The resolution power (RP) is represented by the equation: RP=k.sub.1 .lambda./NA. In usual steppers, the value of the factor k.sub.1 is from 0.7 to 0.8, while in the phase shift mask method, the k.sub.1 factor can be theoretically as low as about 0.35. This phase shift mask still involves many problems in practical use: the problems as below:
1. The thin film forming technique for phase shift film has not been established yet.
2. The CAD for design of a circuit pattern with a phase shift film has not been established.
3. To a certain pattern, the phase shift film is not applicable.
4. The testing method and the conditioning technique have not been established for the phase shift film.
Therefore there are many obstacles in practical use of the phase shift mask, and a long time seems to be required to practicalize the phase shift mask.
Accordingly, in place of the phase shift mask involving many unsolved problems, a projection method for fine pattern image is required which has a resolving power equivalent to or higher than that of the phase shift mask method.
As discussed above, the lithography at the present stage is not capable of forming a pattern as designed when different sizes and different shapes of pattern components are mixedly exist in a pattern. In particular, a random pattern involving patterns of 0.2 .mu.m or finer cannot readily be formed by the lithography.
In fine patterning at a practical use level, enlargement of the focus margin is important. Conventional methods are not capable of forming a patterning on a stepped portion since the focus margin is as small as 0.5 .mu.m for a line-and-space of 0.4 .mu.nm.